Layer transfer materials and processes are known and have many uses in the electronics industry, for example, to make photoresist masks and solder masks, and in the printing industry, for example, to make color reproductions and for proofing color separations. In color proofing processes, light-sensitive layer transfer materials (for example, those of U.S. Pat. No. 4,356,253 and European Patent 243 933 A) are used to make image carriers, and light-insensitive transfer materials are used as color carriers, as disclosed in Frohlich U.S. Pat. No. 4,806,451 or Delaney et al. U.S. Pat. No. 4,902,363.
To prepare for applying colorant onto an image carrier, the surface of the image carrier is exposed in the usual manner, for example, with a light source emitting its light through a negative or a positive onto the image carrier or with a controlled laser beam. The exposed and unexposed areas differ in their ability to accept colorant from a color carrier brought into contact with and then removed from the image carrier. Particularly, in proofing color separations in the printing industry, the color transfer must produce the same quality as in printing. In color printing, which is usually with four colors, each color is not applied areawise, but rather in the form of small dots. The original is screened for this purpose, so that the color dots appear on the image carrier in a regular halftone screen format. The intensity of each color is determined by the size of the color dots. Standard specifications, for example, for a high quality offset print, require a resolution of 2% to 98% dots for a halftone screen width of 60 lines/cm. In this case, it is particularly difficult to achieve satisfactory resolution of the pointy 2% dots in the highlights and of the 98% dots in the shadows. These 2% dots have a diameter of about 27 .mu.m. For good tonal value reproduction, it is also of considerable importance that halftone dots of the same size be sharply defined and uniform over the entire surface area, that is, reproduced without size variations. To achieve satisfactory quality of image reproduction, the colorant must adhere on the image carrier in all areas where colorant is applied. This is especially difficult with small color dots. If very small color dots are missing, this leads to flaws on the image carrier, which distorts the tonal value accuracy of the reproduction.
The invention concerns a layer transfer process for image production, a process with which high uniformity of halftone values can be achieved over the entire surface.
In the case of a layer transfer process of the above-described type, the problem is solved in accordance with the invention by having the contact made at such high pressure that image carrier and/or color carrier are compressed together at least temporarily on mutual facing surfaces so that the sum of the compression depths corresponds to at least the sum of the local peak-to-valley distances of the surface microstructures.
Although the surfaces of image carriers and color carriers feel relatively smooth, closer inspection of the surfaces shows that the surface roughness is relatively high in proportion to the color layer to be applied. At high magnification, the surfaces of image carriers and color carriers look like mountain ranges, wherein the differences between peaks and valleys can reach an order of magnitude of about 10 to 50 .mu.m. During the color transfer, two "valleys" of image carrier and color carrier could lie opposite each other. Consequently, the color in the valley of the color carrier is brought into contact with the image carrier only with light pressure or even without pressure. In an extreme case, there is no contact at all at this site. When the color carrier is removed, the color adheres unsatisfactorily or not at all to the image carrier and produces a flaw. In accordance with the invention, this problem is overcome by the color carrier and image carrier being compressed together with a pressure so high that their surface structures mutually adapt. Depending on the hardness of the materials used, protrusions on the color carrier are pressed on the surface of the image carrier long enough to reach a state wherein the surfaces of the color carrier and the image carrier are in complete contact. Naturally, the image carrier might be hardened in the area where its surface is compressed, so that it becomes harder than the color carrier. With an increase in pressure, the color carrier is also compressed, so that, ultimately in each case, the two surfaces of image carrier and color carrier lie completely on each other, that is, are in contact on each dot, practically speaking. Thus, the color of the color carrier is pressed more evenly onto the image carrier. Consequently, the color deposit on the image carrier is more uniform. In this process, the local peak-to-valley distance is a measure of the difference in height between adjoining "peaks" and "valleys" on the surface of image carriers or color carriers.
Until now, it was considered impossible to accomplish a layer transfer process for image production with such high pressure. Such high pressure, it was feared, would flaw registration. This would make repetitive performance of the process impossible, for example, for the production of four-color prints, because the individual colors could not be positioned accurately. Furthermore, it was feared that such high pressure would cause dot growth, distorting the reproduction of the original. It has now been shown that these concerns are groundless.